Climate study shows nuances in Arctic carbon cycle

Warmer seas don’t always take up more carbon

West Antarctic ice sheets

How will melting sea ice affect global carbon cycles? bberwyn photo.

By Summit Voice

FRISCO — As sea ice inexorably declines, the Arctic Ocean has started to absorb more carbon — by some as estimates, as much as one additional megaton each each, thanks to increased biological productivity.

But those effects are not spread evenly across the region, according to a new study that paints a nuanced picture of how global warming is changing the carbon cycle in the Arctic. The MIT research team modeled changes in Arctic sea ice, temperatures, currents, and flow of carbon from 1996 to 2007, and found that the amount of carbon taken up by the Arctic increased by 1 megaton each year.

But their detailed analysis found that some areas of the Arctic where temperatures have warmed the most are actually storing less carbon. Instead, these regions — including the Barents Sea, near Greenland — have become a carbon source, emitting carbon dioxide to the atmosphere.

While the Arctic Ocean as a whole remains a carbon sink, MIT principal research scientist Stephanie Dutkiewicz says places like the Barents Sea paint a more complex picture of how the Arctic is changing with global warming.

“People have suggested that the Arctic is having higher productivity, and therefore higher uptake of carbon,” Dutkiewicz said. “What’s nice about this study is, it says that’s not the whole story. We’ve begun to pull apart the actual bits and pieces that are going on.”

Ocean carbon cycle explained

As organisms like phytoplankton grow in surface waters, they absorb sunlight and carbon dioxide from the atmosphere, incorporating the carbon into cell walls and other organic structures. When the plankton dies, the materials sink to the bottom, storing carbon. Over time, bacteria eat away at the detritus, converting it back into carbon dioxide that, when stirred up by ocean currents, can escape into the atmosphere.

After modeling the changing Arctic between 1996 and 2007, they found that the ocean stored, on average, about 58 megatons of carbon each year — a figure that increased by an average of 1 megaton annually over this time period.

Dutkiewicz said that result was not surprising, as the Arctic has long been known to be a carbon sink. The group’s results confirm a widely held theory: With less sea ice, more organisms grow, eventually creating a bigger carbon sink.

A new counterbalance?

But after studying regional differences in their models, the researchers found that the ocean took up less carbon in 2007, when sea ice melt was more extensive than in previous years — an unexpected finding, in light of the theory that less sea ice leads to more carbon stored.

The model was able to trace the discrepancy to the Greenland and Barents seas, regions of the Arctic Ocean that take in warmer waters from the Atlantic. (In warmer environments, carbon is less soluble in seawater.)

According to the study, results point to a subtle balance: An ocean’s carbon flow depends on both water temperature and biological activity. In warmer waters, carbon is more likely to be expelled into the atmosphere; in waters with more biological growth — for example, due to less sea ice — carbon is more likely to be stored in ocean organisms.

In short, while the Arctic Ocean as a whole seems to be storing more carbon than in previous years, the increase in the carbon sink may not be as large as scientists had previously thought.

“The Arctic is special in that it’s certainly a place where we see changes happening faster than anywhere else,” Dutkiewicz said. “Because of that, there are bigger changes in the sea ice and biology, and therefore possibly to the carbon sink.”

The paper by Dutkiewicz and co-authors Mick Follows and Christopher Hill of MIT, Manfredi Manizza of the Scripps Institute of Oceanography, and Dimitris Menemenlis of NASA’s Jet Propulsion Laboratory is published in the journal Global Biogeochemical Cycles.

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